 When the nerve impulse reaches the nerve terminal with the release of the neurotransmitter that is the acetylcholine and the neuromuscular junction there is generation of end plate potential and when end plate potential reaches a threshold which always happens in physiological conditions there is generation of action potential. Now this action potential that is the electrical activity on the muscle membrane leads to the generation of mechanical activity. So, excitation contraction coupling is basically the coupling between the excitation that is the action potential on the muscle membrane with the contraction that is the mechanical activity of the muscle. Now for excitation contraction coupling to happen this electrical activity that is the action potential should travel along the muscle membrane fiber and reach to the t-tubules. These t-tubules are basically invaginations of the muscle membrane which dip into the muscle. Now why is it important because we have to link the action potential with the release of the calcium which is responsible for contraction and this calcium should release into the sarcoplasm. So this linking of excitation on contraction is brought by interaction of t-tubules that is the invagination of the muscle membrane with the L-tubules. So here both are L-tubules that is the sarcoplasmic reticulum which is basically smooth endoplasmic reticulum. So t-tubules are basically present transversely that is why the name t-tubules t-po transfers and this smooth endoplasmic reticulum which is present in close contact with the t-tubules is present longitudinally and that is why the term L-tubules and this interaction of t-tubules and L-tubules forms a triad. So there is one t-tubule and two L-tubules and this is known as sarcoplasmic triad and this is responsible for the release of calcium. So how that is happening? On this t-tubules are present receptors known as DHPR receptors, dihydropyridine receptors and on the L-tubules are present another receptors or basically these are channels known as RYR or trinodyne receptors. So when the action potential travels along the membrane what happens that there is physical interaction between this DHPR and RYR and this causes the opening of these RYR. So these are basically ligand gated channels. So this DHPR is voltage gated channels and these voltage gated channels interact with RYR and cause the opening of these rhinodyne receptors and cause the release of calcium from smooth endoplasmic reticulum. So as RYR opens calcium enters into the sarcoplasm. So this smooth endoplasmic reticulum is a huge storehouse of calcium so in a skeletal muscle remember it is a physical interaction between DHPR present on the t-tubules and RYR present on the L-tubules which is responsible for release of the calcium. On the other hand in a smooth muscle and cardiac muscle what happens that in action potential there is entry of calcium. So in a skeletal muscle action potential is due to the entry of sodium ions but in a smooth muscles the depolarization phase is due to the entry of calcium ions and in ventricular action potential, lateral action potential the plateau phase there is entry of calcium ions and this calcium ions basically interact with the rhinodyne receptors which are present on the L-tubules. So that's why I'm saying that these are ligand gated channels and this interaction causes the release of calcium. So that is known as calcium induced calcium release. This calcium induced calcium release is important for the release of calcium in a smooth muscle and cardiac muscle but in a skeletal muscle it is only the intracellular source of calcium that is the calcium present in the smooth endoplasmic reticulum which is released by physical interaction. Fine now this calcium interacts with the troponin and ultimately leads to the contraction of the muscles but for relaxation it is important that this calcium should enter back into the smooth endoplasmic reticulum and that is brought about by a pump known as smooth endoplasmic reticulum calcium ATPase. Again this pump is present on the membrane of smooth endoplasmic reticulum and as soon as calcium is released they start functioning more causing the pumping of calcium back into the smooth endoplasmic reticulum and thus leading to relaxation. So fundamentally it is the release of calcium which is linking the excitation that is the action potential with the mechanical activity that is the contraction. Thanks for watching the video if you like did do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.